What Is Image Compression? How File Size Reduction Actually Works
Image compression is the process of reducing a file's size — usually so it loads faster, uploads more quickly, or takes up less storage — and understanding the basic mechanics behind it makes it much easier to predict which images will compress well and which won't.
The two fundamentally different approaches
Lossless compression shrinks a file by finding more efficient ways to represent the exact same pixel data, without discarding anything — the decompressed result is pixel-for-pixel identical to the original. Lossy compression shrinks a file by intentionally discarding some visual information, in a way designed to be as visually unnoticeable as possible — the decompressed result is an approximation of the original, not an exact copy, but usually one that's genuinely hard to distinguish from it at normal viewing conditions.
Why lossy compression achieves much bigger reductions
Because lossy compression is allowed to actually throw away data (rather than just finding a more efficient way to store the same data), it can achieve far more dramatic size reductions than lossless compression ever could. This is why formats like JPG, which use lossy compression, typically produce much smaller files than a lossless format like PNG for the same photographic image.
Why some images compress better than others
Compression — of either kind — generally works by finding redundancy and predictability in the image data. A photo with smooth gradients and large areas of similar color compresses more efficiently than one with lots of fine, chaotic detail (a close-up photo of grass or gravel, for instance), since there's simply more redundancy for the compression to exploit in the former. This is also why compressing an already-compressed image (re-compressing a JPG, for instance) yields diminishing returns — much of the easily exploitable redundancy is already gone.
The quality setting is a dial, not a switch
For lossy compression specifically, a quality setting controls how much detail gets discarded — higher settings keep more detail at a larger file size, lower settings discard more for a smaller file. There's no single universally correct setting; the right choice depends on the specific image and how closely it'll actually be viewed.
Compress Image lets you directly compare file size at different quality settings before committing to one. The image converter is useful if switching to a more compression-friendly format (like JPG or WEBP for photographic content) would help more than adjusting quality alone.